RTX 3080 Having a Meltdown

You have a great machine, so no worries there. But hopefully when you were getting advice you were informed about how important the type of media is that you work with. The media has a huge impact.

The H264 video codec is not optimized for editing. If it's high res, high bitrate, high framerate, that can all add to the difficulty in your computer decoding (playing) that type of footage. On a computer like that, you could probably push through a decent amount of H264 on its own, and your framerate isn't high, at least on the 25 FPS clip in the MOV container.

I think the problem starts with the addition of the other clip. When you add multiple video sources that are H264, that's twice the decoding the computer has to do. Even though your video card is assisting (as shown in your picture, it's maxing out the decoder portion of the card), you now have two H264 sources, and the second one is high framerate, and not only that it's variable framerate, which causes all kinds of issues for editing software, which likes a constant framerate.

Fixing the Variable Framerate (VFR) usually involves transcoding it in a software that can convert it to constant. Usually something like Handbrake or Shutter Encoder. If you don't need the audio from that file and you just need the video, sometimes you can just use Adobe Media Encoder. The resulting file would be a constant framerate, but things like audio drifting out of sync would get "burned in." -- As a note, VFR is common in screen capture or phone video where the device can't keep up with the recording so drops the framerate until it can catch up.

Addressing the H264 is about optimizing your media. The two ways to do that are two:

1) Transcode your media before working with it to an optimized codec, also called an intermediate codec. The most common of those are Quicktime ProRes, DNxHD/HR, or Cineform. (I usually use ProRes)

2) Make proxies into an optimized video codec. This is typically the route that I go because you can make proxies at any point during a project, and when the project is done you can delete them. I use the Quicktime ProRes Proxy Low/Med resolution presets right in Premiere for most situations. Note that creating proxies for VFR media is not recommended, as the proxy may not line up with the original clip so you don't get an accurate sense of what you're doing with editing.

1) Try to interpret that 51.72fps screen recording footage as 50fps. If not help, 2) Turn off hardware accelerated decoding in Preferences > Media. If not help, 3) Convert that VFR (variable framerate) footage into CFR (constant framerate) via Handbrake 3rd-party tool